Sync with other maps apps (Favorites, Tracks, etc.)

We should work on having these apps sync with us:

• GNOME Maps might become compatible this summer, ref Provide APIs to sync with gnome-maps #30, cc @Sumaid @mlundblad
• Marble would require a few changes to be compatible
• OsmAnd would be nice! Nothing like that yet. Discussion about sync at https://github.com/osmandapp/Osmand/issues/2750#issuecomment-519973845
• Maps.me would be best since they are widely used, but they have a company behind them and already offer their own proprietary favorites backup.
• Organic Maps is a fork of Maps.me without the proprietary features. Discussion about sync at https://github.com/organicmaps/organicmaps/issues/622, and unfortunately they also seem focused on providing their own integrated backup only.
• CoMaps is a community fork of Organic Maps. Discussion about sync at https://codeberg.org/comaps/comaps/issues/262

(Carried over from @eneiluj’s notes in https://github.com/nextcloud/maps/issues/56 )

No repository files, tests, or entry points are identified. Start by reviewing the linked GNOME Maps, OsmAnd, Organic Maps, and CoMaps discussions, then establish which app and sync scope are in bounds; done cannot be determined until a single compatibility target and protocol are agreed.